The present invention relates to a method of adjusting the stroke of a press brake having a movable beam supporting a punch, a fixed beam supporting a die, displacement means for displacement of the movable beam with respect to the fixed beam, the displacement means resting on side frames immovably attached to the fixed beam, linear encoders for measuring the displacement of the movable beam with respect to the side frames, at least one sensor measuring a physical parameter varying with the force exerted by the punch on a metal sheet placed on the die and an electronic control device for controlling the bending movement, controlling the speed of the displacement movement of the movable beam between a top dead centre and a bottom dead centre and provided with a calculation means for correcting the value of the bottom dead centre depending on the measurement of said displacement and said physical parameter.
Patent CH 686119 from the applicant describes a press brake of this type. During bending of a metal sheet, the force suffered by the side frames of a press under the effect of the thrust of the cylinders causes flexing of the side frames, which can result in a deformation of the frame of up to 1-2 mm. This flexing alters the depth of penetration of the punch into the die, which creates an error in the bending angle obtained on the piece to be bent. In the adjustment method according to CH 686119, the force suffered by each of the side frames under the action of the movable beam displacement means is determined, for example by means of pressure sensors, and each of the values obtained is compared with a predetermined diagram establishing the relationship between the force suffered by the respective side frame and the flexing of the side frame, and the stroke of the movable beam is increased so as to compensate for the bending errors due to the deformations of the press, in particular due to the flexing of the side frames.
Another parameter capable of generating an error in the bending angle is the variability in the thickness of the metal sheet processed. The nominal thickness of the metal sheet is one of the parameters entered into the control electronics of the press brake during initial adjustment of the stroke.
In principle, the actual thickness of the metal sheet must be measured each time by the operator. This is because sheet steel manufacturers supply sheets whose actual thickness has variations which may be up to 10% of the nominal value of the thickness. If a metal sheet of 2 mm nominal thickness has, for example, to be bent at 90xc2x0 in a 12 mm V-shaped opening, axc2x110% variation in the thickness will, if it is not corrected, result in a bending angle variation between 88xc2x0 and 92xc2x0.
The operator should therefore, before each bending operation, measure the actual thickness of the metal sheet he is going to process and enter this data into the control electronics of the press brake so that said press brake can correct the stroke of the movable beam.
Experience shows that this preliminary measurement is unfortunately often neglected by the operators, which causes the production of a number of pieces whose bending angle is outside the tolerance limits.
The aim of the present invention is to propose a method of adjusting the stroke of a press brake taking into account the variations in the actual thickness of the metal sheets processed, without requiring intervention from the operator.
To that end, the invention proposes a method of the field as defined above, in which the difference between the actual thickness of the metal sheet and the reference value (e) of the thickness of the metal sheet is measured by the electronic control device of the press, by comparing the actual position of the displacement at which there occurs a predetermined variation of a physical parameter varying with the force exerted by the punch on the metal sheet placed on the die, with the theoretical position of the displacement where this predetermined variation should occur, and in which the electronic control device calculates a correction of the bottom dead centre by taking account of this difference in thickness.
Preferably, in order to detect more precisely said actual displacement position where the variation xcex94p occurs, the speed of the displacement movement is reduced to a measurement acquisition speed (vam), less than a predetermined bending speed (v1), when the punch is at a predetermined distance from the theoretical pinching level of the metal sheetxe2x80x94that is to say, the level reached by the beam during its descent, where the metal sheet starts to be pinched between punch and die, if the thickness of the metal sheet has the reference value (e)xe2x80x94greater than the manufacturing thickness tolerance xcex94e of said sheet, and the speed of the displacement movement increases to said bending speed after detection of said predetermined variation xcex94p of said physical parameter (p).
According to a preferred embodiment of the invention, the displacement of the movable beam is carried out at approach speed from top dead centre to a predetermined safety distance with respect to the theoretical pinching level, the distance where the speed is reduced to a bending speed; the speed of the beam is again reduced to a measurement acquisition speed, at a measurement acquisition distance, that is to say at a distance with respect to this theoretical pinching level equal to or greater than the manufacturing thickness tolerance of the metal sheet; the speed is maintained at this measurement acquisition speed over a displacement distance equal to at least twice the measurement acquisition distance; the displacement speed is then increased again to the bending speed. Finally, it is reduced and brought back to zero at the approach of bottom dead centre (BDC).
While the displacement speed of the movable beam is reduced to the measurement acquisition speed, the system can perform a large number of measurement cycles on the physical parameter per unit length of displacement and can therefore detect precisely the start of the variation of this parameter, as well as the precise position where the variation of the parameter exceeds a predetermined value, which signifies that the punch has come into contact with the metal sheet and is starting to pinch said sheet.
The control electronics determines the actual thickness of the metal sheet while the beam is descending at the reduced measurement acquisition speed and recalculates the bending operation parameters, notably a correction of bottom dead centre (BDC).
Correction of bottom dead centre comprises a geometrical level correction due to the over- or under-thickness of the metal sheet. In addition, the thickness of the metal sheet is a parameter which determines to a great extent the reactive force suffered by the press, notably the side frames, during bending, and, consequently, the deformation of the press and the penetration error of the punch which results therefrom. From preliminary tests, it is possible to produce a chart or algorithm linking actual thickness and deformation of the press and correct the penetration error from the measurement of the actual thickness alone.
It is however possible, during a bending operation, to measure both the actual thickness of the metal sheet when the pinching starts and the reactive force suffered by the press during the bending itself and to carry out the bottom dead centre correction from these two distinct measurements.
Two different parameters can be used for determining the variations in thickness of the metal sheet and the flexing suffered by a side frame.
If, for determining the variations in thickness of the metal sheet, the choice is made of a physical parameter which is also representative of the geometrical deformations of the press, this same parameter makes it possible to correct for both the variations in sheet thickness, by detecting an initial variation xcex94p of the parameter, and the deformation of the press, by measuring the maximum variation of the parameter during bending.
Preferably, the force suffered by the side frames under the action of the beam displacement means is determined during the bending operation by measurement of the same physical parameter and compared with a diagram establishing the relationship between force suffered by a side frame and flexing of said side frame, and the stroke of the movable beam is also increased so as to compensate for the bending errors due to the deformations of the press from this measurement, besides correction of the errors due to variations in thickness of the metal sheet.
If the displacement means of the press brake comprise two hydraulic cylinders associated respectively with two side frames, the value of the physical parameter can be measured at each side frame and the electronic control device can independently correct the stroke of each cylinder to its bottom dead centre, or recalculate a common correction value from these two measurement zones.
The physical parameter chosen can be the hydraulic pressure prevailing in the upper part of a cylinder.
The physical parameter can also be the mechanical stress exerted by a cylinder on the movable beam.
The physical parameter can also be the distance between a point in the top part and a point in the bottom part of the frame of the press.
The physical parameter can also be a mechanical stress suffered by part of the frame of the press.
Other features and advantages of the present invention will emerge in the description below of preferred embodiments, referring to the accompanying drawings.